In a semiconductor module in the related art, after a ceramic circuit board provided with semiconductor elements is joined to a heat sink via a joint layer, such as solder, strain concentration caused by thermal stress occurs at an edge of the joint layer when the temperature is restored to normal temperature from a high temperature at the time of joint. In order to ease the strain concentration, the heat sink is provided with a convex portion having an area smaller than a joint area to the joint layer (see, for example, JP-A-11-265976). Owing to this configuration, strain concentration is eased by increasing a thickness of the joint layer at the edge where the strain concentration occurs.